package demo5;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * 子序列问题
 *      等差数列划分II - 子序列
 */
public class test8 {
    public int numberOfArithmeticSlices(int[] nums) {
        // 创建hash表
        Map<Long, List<Integer>> hash = new HashMap<>();
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            long temp = (long)nums[i];
            if (!hash.containsKey(temp)) {
                hash.put(temp, new ArrayList<Integer>());
            }
            hash.get(temp).add(i);
        }
        // 创建dp表
        int[][] dp = new int[n][n];
        // 初始化

        int sum = 0;
        for (int j = 2; j < n; j++) {
            for (int i = 1; i < j; i++) {
                long a = 2L * nums[i] - nums[j];
                if(hash.containsKey(a)) {
                    for(int k : hash.get(a)) {
                        if(k < i){
                            dp[i][j] += dp[k][i] +1;
                        } else{
                            break;
                        }
                    }
                }
                sum += dp[i][j];
            }
        }
        return sum;
    }
}
